gps basic concepts
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BASIC CONCEPTS OF
GLOBAL POSITIONINGSYSTEMS (GPS)
Introduction to GPS
Merritt College
Marc Epstein, Instructor
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History of Navigation
Early sailors navigated by sun and stars
Until the late 1700s sailors could onlydetermine latitude
Development of accurate seaworthychronometers by the British enabledcalculation of longitude
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Early Electronic Navigation
Radio based system
LORAN-C and DECCA
Triangulation of ground based radio signalsAccurate to 900 1000 feet
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What is GPS?
The Global Positioning System (GPS) is asatellite based navigation system made upof a network of 24 satellites placed into
orbit by the U.S. Department of Defensebeginning in 1976.
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How is GPS Used in Society?
GPS technology is used by any individual or groupwhich needs accurate location data.
Aeronautic Navigation First Responder Emergency ManagementGround Transportation Management Land Use Management Law Enforcement
Marine NavigationNatural Resource ManagementUtility Management
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Why GPS?
GPS was originally intended for militaryapplications, but in the late 1980s thegovernment made the system available for
civilian use.
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Is GPS Restricted?
GPS works in any weather conditions,anywhere in the world. 24 hours a day.There is no subscription fee or setup
charges to use GPS.
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The Satellites
The 24 GPS satellitescircle the earth twiceeach day in a veryprecise orbit andtransmit signalinformation to the
earth.
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About the Satellites
The GPS satellites are orbiting the earthabout 12,000 miles above the surface.
The GPS satellites are constantly moving,making two orbits in 24 hours at speedsroughly 7,000 miles per hour.
The GPS satellites are powered with solarenergy with backup batteries.
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The GPS Unit
The GPS Unitreceives the signalsfrom the satellites to
determine locationand conduct otherfunctions
programmed intothe unit.
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Determining LocationThe GPS Unit calculates location
based upon geometry and time.
GPS triangulation really istrilateration since it uses
geometry rather than angles.The GPS unit measures distance
through the time it takes for theradio signals to be received from
the satellites.The radio waves from the
satellites travel at the speed oflight (186,000 miles/second)
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Calculation of Location
The GPS unit is programmed withinformation on the orbits of the satellites.
The GPS unit calculates the time it takesfor each of the satellite signals to reachthe surface to determine distance.
The location on the surface is the pointwhere the GPS unit calculates where allthe distances intercept.
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The GPS Signal
GPS satellites transmit two low power radiosignals designated L1 and L2. Civilian GPSuses the L1 frequency of 1575.42 MHz in
the UHF band. The signals travel by line ofsight, meaning they pass through clouds,glass, and plastic but will not pass through
most solid objects such as buildings andmountains.
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Whats in the GPS Signal
Pseudorandom Code
Ephemeris Data
Almanac Data
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Pseudorandom Code
ID code that identifies which satellite istransmitting information. Many GPS unitswill provide information on which satellites it
is receiving.
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Ephemeris Data
Provides the GPS unit with the informationon where each GPS satellite should be atany time throughout the day. Each satellite
transmits ephemeris data showing orbitalinformation for that satellite and every othersatellite in the system.
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Almanac Data
Contains information about the status of thesatellite, current date and time. This part ofthe signal is essential for determining
position.
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Signal Error
Atmospheric conditions
Signal multi-path
Receiver clock errorsOrbital errors
Number of satellites visible
Satellite geometryIntentional signal degradation
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Atmospheric Conditions
The satellite signal slows as it passesthrough the ionosphere and tropospherelayers of the atmosphere. The GPS system
uses a built in model that calculates anaverage amount of delay to partially correctfor this type of error.
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Signal Multi-path
This occurs when the GPS signal is reflectedoff of objects such as tall buildings or largerock surfaces before it reaches the receiver.
This increases travel time of the signal,thereby causing errors.
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Receiver Clock Error
A receivers built-in clock is not as accurateas the atomic clocks onboard the GPSsatellites. Therefore, it may have very slight
timing errors.
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Orbital Errors
Also known as ephemeris errors, these areinaccuracies of the satellites reportedlocation.
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Number of Satellites Visible
The more satellites the GPS unit can receive,the better the accuracy. Buildings, terrain,electronic interference, or sometimes even
dense foliage can block signal reception,causing position errors or possibly no readingat all.
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Intentional Signal Degradation
Selective availability (SA) is an intentionaldegradation of the signal once imposed bythe Department of Defense. SA was
intended to prevent military adversariesfrom using the highly accurate GPS signals.The government turned off SA in May 2000.
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DGPS
Differential GPS
Ground stations which transmit true position
WAAS Wide Area Augmentation SystemDGPS increases accuracy
Non WAAS enabled units accurate to 30 feet
WAAS enabled units accurate to 15 feet
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The GPS Unit Functions
Display of Position
Marking Waypoints
TracksCompass
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Display of Position
The GPS Unit can display position in severalcoordinate systems
In order to make position data consistentwith GIS software set the units to display inlatitude and longitude
Use the degrees decimal option
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Waypoints
A waypoint is any point that you store inmemory
Waypoints are identified with unique
numbersPrior to recording a waypoint allow the unit
time to average the position for greater
accuracyThe time you use to average your position is
dependent on the estimated accuracy
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Tracks
The tracks feature creates an electronic trailas you move
It records and saves your movements
It allows you to retrace your path
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Compass
The compass page uses traditionalnavigation to display your current directionof travel
This is not a magnetic compass
It only provides readings with motion
Most GPS compass pages will providebearing and current speed
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Accuracy, Precision, andResolution
Accuracy Relationship to True Value
Precision Relationship Between DataPoints
Resolution - Smallest increment the GPSunit can measure or ability to distinguishbetween different points
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Recording GPS Data Points
When recording GPS data points you wantto use a spread sheet format
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The Data Sheet Columns
Waypoint or identification number
Latitude
LongitudeDescriptive data
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Improving Accuracy
Check satellite geometry
Further apart the satellites the greater theaccuracy
Increase height
If you are in a canyon or under canopycover increasing height may increase signalor increase number of satellites beingreceived
Use multiple GPS units and average readings
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Field Collection Tips
Object which you can not reach the centerCollect data points at corners and averagethe readings to get center point
When collecting data points where the dotswill be connected by GIS software Collectdata points for corners and in betweencorners
Pay attention to waypoint numbers if youare recording the waypoints in the GPS unit
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Support the Data Inputter
Write neatly
Sketch maps
Provide unit numberProvide names of data collectors
Make notes of any special features or
problems